G. Bhagavannarayana

Bio: G. Bhagavannarayana is an academic researcher from National Physical Laboratory. The author has contributed to research in topics: Crystal & Single crystal. The author has an hindex of 35, co-authored 209 publications receiving 5919 citations. Previous affiliations of G. Bhagavannarayana include Anna University & Annamalai University.

A high-resolution diffuse X-ray scattering study of defects in dislocation-free silicon crystals grown by the float-zone method and comparison with Czochralski-grown crystals

Abstract: Point-defect aggregates in (111) dislocation-free silicon single crystals grown by the float-zone (FZ) method have been studied by diffuse X-ray scattering (DXS) and compared with those in the Czochralski-grown (CZ) crystals. A two-axis X-ray diffractometer was used. It employs three monochromators in (+, −, −) setting to obtain a highly collimated and monochromatic Mo Kα1 beam. DXS measurements were made around the 111 reciprocal-lattice point (r.l.p.) with K* along ±[111] and ±[01{\bar 1}]; K is the vector which joins the elemental volume of the reciprocal space under investigation to the nearest r.l.p. For FZ crystals for a given K* the DXS intensity was higher for θ θB showing that the anisotropy (DXS Iθ > θB − DXS Iθ < θB) is negative, as expected for vacancy clusters. For CZ crystals the anisotropy was positive, owing to the presence of interstitial clusters. The magnitude of anisotropy in the FZ crystals was smaller than that observed in the CZ crystals. The DXS intensity varies approximately as K−2 near Bragg peaks (Huang scattering) and as K*−4 (Stokes–Wilson scattering) away from it. From the K* values where the changeover from Huang to Stokes–Wilson scattering takes place the size of the clusters assumed to be the origin of the observed DXS is estimated as ~ 2 × 10−4 and 2.6 × 10−3 mm for FZ and ~ 5.5 × 10−4 and 3 × 10−3 mm for CZ crystals. The experimental data were compared with theoretically calculated DXS distributions assuming the defects to be dislocation loops. The number of point defects in a loop has been estimated.

A study of the effect of annealing on Fe‐doped LiNbO3 by HRXRD, XRT and FT–IR

Abstract: The annealing effect on the structural perfection of Fe-doped LiNbO3 single crystals has been studied by high-resolution X-ray diffractometry (HRXRD), X-ray topography (XRT) and Fourier transform infrared (FT–IR) spectroscopy. The single crystals, prepared by mixing Li2CO3 and Nb2O5 powders in the molar ratio 48.6:51.4 with 0.05 mol% of iron at 1415 (1) K, were grown by the Czochralski (CZ) method along the [001] direction in air and poled during crystal growth by the application of a DC field. Two low-angle (tilt angle ∼1 arc minute) structural grain boundaries were observed in as-grown specimens. FT–IR spectra revealed that these crystals contain OH− and CO32− ionic defects. Grain boundaries and CO32− ionic defects were successfully removed, while the concentration of OH− ions was considerably reduced by post-growth thermal annealing at elevated temperatures.

An Interesting Correlation between Crystalline Perfection and Second Harmonic Generation Efficiency on KCl-and Oxalic Acid-Doped ADP Crystals

Abstract: The effect of dopants (over a concentration range from 1 to 10 mol %), namely KCl and oxalic acid (C2H2O4·2H2O, Oxa), on the growth process, crystalline perfection, and nonlinear optical (NLO) properties of ammonium dihydrogen phosphate (NH4H2PO4, ADP) single crystals grown by a slow evaporation solution growth technique has been investigated. The high-resolution X-ray diffraction (HRXRD) studies used to evaluate crystalline perfection reveal some interesting features on the ability of accommodating the dopants by the crystalline matrix. The remarkable and systematic increase in the broadness and asymmetry of the diffraction curves as the concentration increases clearly indicates that the dopants predominantly occupied the interstitial positions in the crystalline matrix. Due to the larger size of the Oxa molecule, the crystalline matrix could not accommodate these dopants, which led to the formation of a very low angle (tilt angle <1′) internal structural grain boundaries at high concentrations. The rela...

Enhancement of crystalline perfection by organic dopants in ZTS, ADP and KHP crystals as investigated by high‐resolution XRD and SEM

Abstract: To reveal the influence of complexing agents on crystalline perfection, tristhiourea zinc(II) sulfate (ZTS), ammonium dihydrogen phosphate (ADP) and potassium hydrogen phthalate (KHP) crystals grown by slow-evaporation solution growth technique using low concentrations (5 x 10 -3 M) of dopants like ethylenediamminetetraacetic acid (EDTA) and 1,10-phenanthroline (phen) were characterized by high-resolution X-ray diffractometry (XRD) and scanning electron microscopy (SEM). High-resolution diffraction curves (DCs) recorded for ZTS and ADP crystals doped with EDTA show that the specimen contains an epilayer, as observed by the additional peak in the DC, whereas undoped specimens do not have such additional peaks. On etching the surface layer, the additional peak due to the epilayer disappears and a very sharp DC is obtained, with full width at half-maximum (FWHM) of less than 10 arcsec, as expected from the plane wave dynamical theory of X-ray diffraction for an ideally perfect crystal. SEM micrographs also confirm the existence of an epilayer in doped specimens. The ZTS specimen has a layer with a rough surface morphology, having randomly oriented needles, whereas the ADP specimen contains a layer with dendric structure. In contrast to ADP and ZTS crystals, the DC of phen-doped KHP shows no additional peak, but it is quite broad (FWHM = 28 arcsec) with a high value of integrated intensity, p (area under the DC). The broadness of the DC and the high value of p indicate the formation of a mosaic layer on the surface of the crystal. However, similar to ADP and ZTS, the DC recorded after etching the surface layer of the KHP specimen shows a very sharp peak with an FWHM of 8 arcsec. An SEM photograph of phen-doped KHP shows deep cracks on the surface, confirming the mosaicity. After removing the surface layer, the SEM pictures reveal a smooth surface. A similar trend is observed with other complexing agents, like oxalic acid, bipy and picolinic acid. However, only typical examples are described in the present article where the effects were observed prominently. The investigations on ZTS, ADP and KHP crystals, employing high-resolution XRD and SEM studies, revealed that some organic dopants added to the solution during the growth lead to the formation of a surface layer, due to complexation of these dopants with the trace metal ion impurities present in the solution, which prevents the entry of impurities, including the solvent, into the crystal, thereby assisting crystal growth with high crystalline perfection. The influence of organic dopants on the second harmonic generation efficiency is also investigated.

Enhancement of SHG efficiency by urea doping in ZTS single crystals and its correlation with crystalline perfection as revealed by Kurtz powder and high‐resolution X‐ray diffraction methods

Abstract: The enhancement of second-harmonic generation (SHG) efficiency by urea doping in tristhioureazinc(II) sulfate (ZTS) single crystals and its correlation with crystalline perfection have been investigated. ZTS is a potential semiorganic nonlinear optical material. Pure and urea-doped single crystals of ZTS have been successfully grown by the slow evaporation solution technique. The presence of dopant has been confirmed and analysed by Fourier transform infrared spectrometry. The influence of urea doping at different concentrations on crystalline perfection has been thoroughly assessed by high-resolution X-ray diffractometry (HRXRD). HRXRD studies revealed that the ZTS crystals could accommodate urea up to a critical concentration without any deterioration in crystalline perfection. Above this concentration, very low angle structural grain boundaries developed and it seems the excess urea above the critical concentration was segregated along the grain boundaries. At very high doping concentrations, the crystals were found to contain mosaic blocks. The SHG efficiency has been studied using the Kurtz powder technique. The relative SHG efficiency of the crystals was found to increase substantially with the increase in urea concentration. The correlation found between crystalline perfection and SHG efficiency is discussed.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

Abstract: Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Organic Triplet Sensitizer Library Derived from a Single Chromophore (BODIPY) with Long-Lived Triplet Excited State for Triplet–Triplet Annihilation Based Upconversion

Abstract: Triplet-triplet annihilation (TTA) based upconversions are attractive as a result of their readily tunable excitation/emission wavelength, low excitation power density, and high upconversion quantum yield. For TTA upconversion, triplet sensitizers and acceptors are combined to harvest the irradiation energy and to acquire emission at higher energy through triplet-triplet energy transfer (TTET) and TTA processes. Currently the triplet sensitizers are limited to the phosphorescent transition metal complexes, for which the tuning of UV-vis absorption and T(1) excited state energy level is difficult. Herein for the first time we proposed a library of organic triplet sensitizers based on a single chromophore of boron-dipyrromethene (BODIPY). The organic sensitizers show intense UV-vis absorptions at 510-629 nm (e up to 180,000 M(-1) cm(-1)). Long-lived triplet excited state (τ(T) up to 66.3 μs) is populated upon excitation of the sensitizers, proved by nanosecond time-resolved transient difference absorption spectra and DFT calculations. With perylene or 1-chloro-9,10-bis(phenylethynyl)anthracene (1CBPEA) as the triplet acceptors, significant upconversion (Φ(UC) up to 6.1%) was observed for solution samples and polymer films, and the anti-Stokes shift was up to 0.56 eV. Our results pave the way for the design of organic triplet sensitizers and their applications in photovoltaics and upconversions, etc.

A study of the effect of annealing on Fe‐doped LiNbO3 by HRXRD, XRT and FT–IR

Abstract: The annealing effect on the structural perfection of Fe-doped LiNbO3 single crystals has been studied by high-resolution X-ray diffractometry (HRXRD), X-ray topography (XRT) and Fourier transform infrared (FT–IR) spectroscopy. The single crystals, prepared by mixing Li2CO3 and Nb2O5 powders in the molar ratio 48.6:51.4 with 0.05 mol% of iron at 1415 (1) K, were grown by the Czochralski (CZ) method along the [001] direction in air and poled during crystal growth by the application of a DC field. Two low-angle (tilt angle ∼1 arc minute) structural grain boundaries were observed in as-grown specimens. FT–IR spectra revealed that these crystals contain OH− and CO32− ionic defects. Grain boundaries and CO32− ionic defects were successfully removed, while the concentration of OH− ions was considerably reduced by post-growth thermal annealing at elevated temperatures.

Tailoring the structural, morphological, optical and dielectric properties of lead iodide through Nd 3+ doping

Abstract: Hexagonal single crystal nanosheets of Nd3+ doped PbI2 were effortlessly synthesized via microwave-assisted technique under a power of 700 W and in a duration of 15 minutes with a homogeneous morphology. X-ray diffraction, energy dispersive X-ray spectroscope, scanning electron microscope, FT-Raman, UV-Visible, photoluminescence and dielectric measurement were employed to study the product. High purity, single phase and presence of Nd3+ doping was confirmed. SEM study confirm the formation of nanorods and single crystal nanosheets of very few nanometers in size. Robust vibrational analysis has been carried out and the observed bands are assigned to the vibration modes of E21, A11, A12, 2E21 and 2E11, respectively. These bands are red-shifted when compare to the corresponding bulk values which indicate relaxed nanostructure formation and occurrence of confinement effect. The thickness of the synthesized single crystal nanosheets are found to be in the range of ~20 to 30 nm. The energy band gap was calculated and found to be 3.35, 3.34, 3.42 and 3.39 eV for pure, 1, 3 and 5% Nd3+ doped lead iodide, respectively. The clear blue luminescence has been observed at 440 nm and 466 nm when excited at 250 nm and 280 nm respectively. Dielectric and ac electrical conductivity was also measured and discussed.